Aluminum alloy sheet for press forming, process for manufacturing same, and press-formed product thereof

ABSTRACT

An aluminum alloy sheet for press forming includes an aluminum alloy including 0.4 to 1.5 mass % of Si and 0.3 to 1.0 mass % of Mg, with the remainder being Al and inevitable impurities. With respect to diagonal lengths of indentations formed therein with a Vickers hardness tester, a proportion P (%) of a difference ΔL between a length L0 of a diagonal which forms an angle of 0° with a rolling direction and a length L45 of a diagonal which forms an angle of 45° with the rolling direction to the L0 is 2.0% or less.

TECHNICAL FIELD

The present invention relates to an aluminum alloy sheet for use in press forming, processes for producing the sheet, and a press-formed article thereof.

BACKGROUND ART

In an attempt to attain weight reduction in machines and apparatus for transportation such as motor vehicles, more lightweight aluminum alloy materials have been developed for exterior-material applications and are being put to practical use in place of steel materials which have been conventionally used.

Of such aluminum alloys (hereinafter referred to also as “Al alloys”), Al—Mg—Si aluminum alloys of 6000 series, which are excellent in terms of strength and corrosion resistance, are being investigated as the materials of automotive exterior sheet materials for bodies, doors, fenders, etc.

Automotive exterior parts are generally produced by press forming and, hence, Al alloy sheet materials are required to have excellent press formability. In Patent Documents 1 and 2, investigations are being made on such Al alloy materials for automotive exterior from the standpoint of press formability.

Patent Document 1 discloses an Al alloy sheet for forming which is a 6000-series alloy and in which the intermetallic compounds have specified values of its diameter and number density.

Patent Document 2 discloses an Al alloy sheet which is a 6000-series alloy and in which the internal texture of the sheet material has been specified.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: JP-A-2003-221637

Patent Document 2: JP-A-2009-173972

SUMMARY OF THE INVENTION Problem that the Invention is to Solve

In recent years, Al alloy sheets are required to have better stretch formability so that the sheets are applicable also to press forming for producing exterior materials having a shape with a three-dimensional depth or a complicated shape. It is difficult to sufficiently satisfy such requirement by the techniques disclosed in the above Patent Documents.

The present invention has been achieved under the circumstances described above, and an object thereof is to provide an aluminum alloy sheet for press forming which has excellent press formability and is applicable to deep press forming and to further provide a press-formed article thereof. Another object thereof is to provide processes for producing the aluminum alloy sheet for press forming which has excellent press formability.

Means for Solving the Problems

The present inventors made investigations not only on Al alloy sheet compositions but also on the texture structures, etc. of alloy sheets in order to improve stretch formability. As a result, the inventors thought that it is important for an Al alloy sheet to have no directional dependence of stretchability, in other words, to have excellent isotropy during forming, so that the sheet material is capable of accommodating to press forming in which the sheet material is stretched in any direction.

The inventors have made investigations on conditions for producing a rolled Al alloy sheet having isotropy during forming. As a result, the inventors have found that by causing a sheet in the state of having an accumulated strain to undergo fine recrystallization in an annealing step conducted after the hot rolling step, the anisotropy of the grain structure within the sheet material can be eliminated. The inventors have further found that it is possible to obtain an Al alloy sheet which retains the isotropy in formability even after succeeding steps.

The inventors have found that a ratio between the diagonal lengths of indentations formed with a Vickers hardness tester is effective as an index of the isotropy of the formability of Al alloy sheets.

Moreover, Al alloy sheets having excellent isotropy during forming were found to have a large forming height resulting from stretch forming, to be low in earing and be less apt to show ridging marks. The inventors furthermore found that such Al alloy sheets are excellent also in terms of bake hardenability (BH property), which is the property of improving in strength or proof stress through an artificial aging treatment, e.g., paint baking, performed after press forming. The present invention has been thus accomplished.

That is, the aluminum alloy sheet for press forming in the present invention includes an aluminum alloy including 0.4 to 1.5 mass % of Si and 0.3 to 1.0 mass % of Mg, with the remainder being Al and inevitable impurities, in which, with respect to diagonal lengths of indentations formed therein with a Vickers hardness tester, a proportion P (%) of a difference ΔL between a length L0 of a diagonal which forms an angle of 0° with a rolling direction and a length L45 of a diagonal which forms an angle of 45° with the rolling direction to the L0 is 2.0% or less. In addition, the aluminum alloy preferably includes 0.6 to 1.3 mass % of Si and 0.3 to 0.8 mass % of Mg, with the remainder being Al and inevitable impurities.

According to the configuration, the inclusion of given amounts of Si, Mg, etc. makes it possible to form aging precipitates which contribute to solid-solution hardening and to strength improvement during an aging treatment at a low temperature, thereby improving tensile strength, etc. Furthermore, since the Al alloy sheet satisfies the requirement concerning the diagonal lengths of indentations formed with a Vickers hardness tester, this Al alloy sheet has isotropy during forming and shows excellent press formability.

In addition, with regard to the aluminum alloy in the aluminum alloy sheet for press forming in the present invention, it is possible to include 1.0 mass % or less of Cu, to include at least one of 0.5 mass % or less of Fe and 0.5 mass % or less of Mn, to include at least one of 0.3 mass % or less of Cr, 0.3 mass % or less of Zr, and 0.3 mass % or less of Ti, and to control the content of Zn to 0.5 mass % or less.

According to the configuration, the formability can be further improved.

The process for producing an aluminum alloy sheet for press forming in the present invention includes the following steps in this order: a homogenizing heat treatment step of subjecting a slab of the aluminum alloy having the above composition to a homogenizing heat treatment; a hot rolling step of subjecting to hot-rolling under the condition that a hot rolling ending temperature is 300° C. or lower; an annealing step of subjecting to annealing at a temperature of 300 to 500° C.; a cold rolling step of subjecting to cold-rolling at a cold rolling ending temperature of 100° C. or lower; a solution treatment step of treating at a temperature of 500° C. or higher; and a heating step of heating to a temperature of 70° C. or higher.

The process for producing an aluminum alloy sheet for press forming in the present invention includes the following steps in this order: a homogenizing heat treatment step of subjecting a slab of the aluminum alloy having the above composition to a homogenizing heat treatment; a hot rolling step of subjecting to hot-rolling; a first cold rolling step of subjecting to cold-rolling at a cold rolling ending temperature of 100° C. or lower; an intermediate annealing step of subjecting to intermediate annealing at a temperature of 300 to 500° C.; a second cold rolling step of subjecting to cold-rolling at a cold rolling ending temperature of 100° C. or lower; a solution treatment step of treating at a temperature of 500° C. or higher; and a heating step of heating to a temperature of 70° C. or higher. In addition, the aluminum alloy preferably includes 0.6 to 1.3 mass % of Si and 0.3 to 0.8 mass % of Mg, with the remainder being Al and inevitable impurities.

In the aluminum alloy in the process for producing an aluminum alloy sheet for press forming, it is possible to include 1.0 mass % or less of Cu, to include at least one of 0.5 mass % or less of Fe and 0.5 mass % or less of Mn, to include at least one of 0.3 mass % or less of Cr, 0.3 mass % or less of Zr, and 0.3 mass % or less of Ti, and to control the content of Zn to 0.5 mass % or less.

According to the production processes having the configurations described above, an Al alloy sheet for press forming which has excellent isotropy during forming can be produced from an Al alloy having the above composition.

By press-forming the Al alloy sheet for press forming in the present invention, an Al alloy press-formed article can be obtained.

Advantageous Effects of the Invention

The Al alloy sheet for press forming in the present invention is applicable also to deep press forming and is low in earing and excellent also in terms of inhibition of ridging marks. The processes for producing the Al alloy sheet for press forming in the present invention are capable of producing an isotropic Al sheet which shows excellent formability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart which shows production steps of a first embodiment of the processes for producing an aluminum alloy sheet for press forming in the present invention.

FIG. 2 is a flowchart which shows production steps of a second embodiment of the processes for producing an aluminum alloy sheet for press forming in the present invention.

FIG. 3 is a schematic view for illustrating a method for measuring the length L0 of a diagonal that forms an angle of 0° with the rolling direction and the length L45 of a diagonal that forms an angle of 45° with the rolling direction, among the diagonals of indentations formed with a Vickers hardness tester.

FIG. 4 is a schematic view for illustrating a method for measuring the length L0of the diagonal that forms an angle of 0° with the rolling direction, between the diagonals of an indentation formed with a Vickers hardness tester.

FIG. 5 is a schematic view for illustrating a method for measuring the length L45 of the diagonal that forms an angle of 45° with the rolling direction, between the diagonals of an indentation formed with a Vickers hardness tester.

FIG. 6 is a cross-sectional view for illustrating a test method used in a stretch formability test.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

The aluminum alloy sheet for press forming in the invention and the processes for producing the sheet are explained below on the basis of specific embodiments.

The Al alloy which constitutes the Al alloy sheet for press forming in the invention has a composition that contains 0.4 to 1.5 mass % of Si and 0.3 to 1.0 mass % of Mg, with the remainder being Al and inevitable impurities.

Each of the elements which constitute the Al alloy in the present invention and the contents thereof are explained below.

Si: 0.4 to 1.5 Mass %

Si, along with Mg, is capable of forming aging precipitates which contribute to solid-solution hardening and to strength improvement during an artificial aging treatment at a low temperature, such as paint baking, and is an indispensable element for imparting the strength (proof stress) required for the exterior panels of motor vehicles. In a case where the content of Si is less than 0.4 mass %, the formation of aging precipitates is insufficient, resulting in a decrease in paint baking hardenability (strength). Meanwhile, in a case where the content of Si exceeds 1.5 mass %, coarse crystallized particles and precipitates are formed to reduce press formability and weldability. Consequently, the content of Si is 0.4 to 1.5 mass %, preferably 0.6 to 1.3 mass %.

Mg: 0.3 to 1.0 Mass %)

Mg, along with Si, is capable of forming aging precipitates which contribute to solid-solution hardening and to strength improvement during an artificial aging treatment at a low temperature, such as paint baking, and is an indispensable element for imparting the strength (proof stress) required for the exterior panels of motor vehicles. In a case where the content of Mg is less than 0.3 mass %, the formation of aging precipitates is insufficient, resulting in a decrease in paint baking hardenability (strength). Meanwhile, in a case where the content of Mg exceeds 1.0 mass %, coarse crystallized particles and precipitates are formed to reduce press formability and weldability. Consequently, the content of Mg is 0.3 to 1.0 mass %, preferably 0.3 to 0.8 mass %.

Each of the elements Cu, Fe, Mn, Cr, Zr, Ti, and Zn, which are explained below, has various peculiar functions although they are not indispensable elements. These elements can hence be suitably added and used in accordance with applications or purposes so long as the content of each element does not exceed the upper limit.

Cu: 1.0 Mass % or Less

Cu has the effect of accelerating the formation of aging precipitates in an artificial aging treatment conducted under the condition of a relatively low temperature and short period, and the Cu in a dissolved state is an element capable of improving formability. From the standpoint of expecting these effects, it is preferable that the content of Cu should be 0.1 mass % or higher. Meanwhile, in a case where the content of Cu exceeds 1.0 mass %, stress corrosion cracking resistance, filiform corrosion resistance and weldability are considerably deteriorated. Consequently, in the case where Cu is contained, the content of Cu is 1.0 mass % or less, preferably 0.1 to 0.8 mass %.

Fe: 0.5 Mass % or Less

Fe, along with Mn, forms crystallized particles of an FeMnAl₆ or AlMnFeSi phase or the like during casting or a homogenizing heat treatment, and serves as nuclei for recrystallization during hot rolling and in a final solution treatment. Fe hence is an element which is effective for refinement of recrystallized grains and forming a random texture. In a case where the content of Fe exceeds 0.5 mass %, coarse crystallized particles are formed, resulting in reduced press formability. Consequently, in the case where Fe is contained, the content of Fe is 0.5 mass % or less, preferably 0.1 to 0.3 mass %.

Mn: 0.5 Mass % or Less

Mn, along with Fe, forms crystallized particles of an FeMnAl₆ or AlMnFeSi phase or the like during casting or a homogenizing heat treatment, and serves as nuclei for recrystallization during hot rolling and a final solution treatment. Mn hence is an element which is effective for refinement of recrystallized grains and forming a random texture. In a case where the content of Mn exceeds 0.5 mass %, coarse crystallized particles are formed, resulting in reduced press formability. Consequently, in the case where Mn is contained, the content of Mn is 0.5 mass % or less, preferably 0.1 to 0.4 mass %.

Cr: 0.3 Mass % or Less

Cr is an element which forms dispersoids during a homogenizing heat treatment and has the function of refining grains. In a case where the content of Cr exceeds 0.3 mass %, coarse intermetallic compounds are formed, resulting in decreases in press formability and corrosion resistance. Consequently, in the case where Cr is contained, the content of Cr is 0.3 mass % or less, preferably 0.01 to 0.2 mass %.

(Zr: 0.3 mass % or less)

Zr is an element which forms dispersoids during a homogenizing heat treatment and has the function of refining grains. In a case where the content of Zr exceeds 0.3 mass %, coarse intermetallic compounds are formed, resulting in decreases in press formability and corrosion resistance. Consequently, in the case where Zr is contained, the content of Zr is 0.3 mass % or less, preferably 0.05 to 0.2 mass %.

Ti: 0.3 Mass % or Less

Ti is an element which enables the slab to have fine grains and improves press formability. In a case where the content of Ti exceeds 0.3 mass %, coarse crystallized particles are formed, resulting in reduced press formability. Consequently, in the case where Ti is contained, the content of Ti is 0.3 mass % or less, preferably 0.01 to 0.2 mass %.

Zn: 0.5 Mass % or Less

In a case where the content of Zn exceeds 0.5 mass %, coarse intermetallic compounds are formed and the aluminum alloy sheet has reduced formability and considerably reduced corrosion resistance. Consequently, the content of Zn is regulated to 0.5 mass % or less.

Inevitable Impurities

Inevitable impurities other than Cu, Fe, Mn, Cr, Zr, Ti, and Zn, which are shown above, are supposed to be elements such as Sn, Sc, Ni, C, In, Na, Ca, V, Bi and Sr. These elements are each permitted to be contained unless the features of the present invention are lessened thereby. Specifically; it is preferable that the total content of Cu, Fe, Mn, Cr, Zr, Ti, Zn, and inevitable impurities should be 1.0 mass % or less.

Next, the properties required for the Al alloy sheet for press forming, which is constituted of the Al alloy, are explained below.

Ratio Between the Diagonal Lengths of Indentations

With respect to Vickers hardness, it is the measuring method for measuring the hardness of a metallic material as described in JIS Z2244. In the test, a diamond indenter of a square-based pyramid shape is pushed against the test surface of a specimen at a given test load, and the hardness of the specimen is determined from the size of the resultant indentation (depression). The indentation, in a plan view thereof, is substantially square, and there are two diagonals therein.

In the present invention, a difference in diagonal length between different angles with respect to the rolling direction in a Vickers hardness tester is used as an index of the isotropy in formability of the Al alloy sheet.

Specifically, with respect to the diagonal lengths of indentations formed with a Vickers hardness tester in the Al alloy sheet for press forming, the proportion P (%) of the difference ΔL between the length (L45) of the diagonal of an indentation which forms an angle of 45° or −45° (135°) with the rolling direction and the length (L0) of the diagonal of an indentation which forms an angle of 0° or 90° with the rolling direction to the length (L0) of the diagonal of the indentation which forms an angle of 0° or 90° with the rolling direction is determined.

This calculation is shown by the following mathematical expression.

P(%)=100×|L45-L0|L0=100×ΔL/L0  (1)

In the expression, |45-L0| indicates the difference (absolute value) between L45 and L0.

With respect to the diagonal lengths of indentations formed with a Vickers hardness tester, the length (L0 ) of the diagonal of an indentation which forms an angle of 0° or 90° with the rolling direction is often referred to simply as “length L0 of a diagonal which forms an angle of 0° with the rolling direction”.

Similarly, with respect to the diagonal lengths of indentations formed with a Vickers hardness tester, the length (L45) of the diagonal of an indentation which forms an angle of 45° or −45° (135°) with the rolling direction is often referred to simply as “length L45 of a diagonal which forms an angle of 45° with the rolling direction”.

In the present invention, it is necessary that the proportion P should be 2.0% or less.

Namely, it is necessary that, with respect to the diagonal lengths of indentations formed therein with a Vickers hardness tester, the proportion P (%) of the difference ΔL between the length L0 of a diagonal which forms an angle of 0° with the rolling direction and the length L45 of a diagonal which forms an angle of 45° with the rolling direction to the L0 should be 2.0% or less.

In a case where the proportion P exceeds 2.0%, this Al alloy sheet has high anisotropy in formability, making it difficult to attain a large forming height in stretch forming.

For obtaining the proportion P of 2.0% or less, it is necessary that an Al alloy having the specific composition described above and the specific production conditions described below should be employed to produce an Al alloy sheet in which the anisotropy of the inner grain structure has been eliminated.

A method for measuring the diagonal lengths of indentations is as follows. Indentations with a Vickers hardness tester are formed in a substantially width-direction central area of a sample along the rolling direction (RD direction). At least three indentations in each of which the diagonals form an angle of 0° (90°) with the rolling direction and at least three indentations in each of which the diagonals form an angle of 45° (−45°) therewith are formed. The surface on which indentations are impressed may be either any of the surfaces of the Al alloy sheet or a cross-section of the Al alloy sheet.

The diagonal length of an indentation is determined by photographing a plurality of indentations from above using a microscope, measuring the two diagonal lengths of each of the indentations on the plan-view image obtained, and averaging the measured values. The load of the Vickers hardness tester can be suitably set in accordance with the hardness of the sample.

Processes for producing the Al alloy sheet for press forming in the present invention are explained below.

The production processes in the present invention have a prominent feature in which a sheet in the state of having an accumulated strain is caused to undergo fine recrystallization in an annealing step conducted after the hot rolling step, thereby eliminating the anisotropy of the grain structure within the sheet material.

There are the following two embodiments of the processes for producing the Al alloy sheet for press forming in the present invention. FIG. 1 is a flowchart which shows production steps of a first embodiment of the process for producing an aluminum alloy sheet for press forming in the present invention. FIG. 2 is a flowchart which shows production steps of a second embodiment of the process for producing an aluminum alloy sheet for press forming in the present invention.

First Embodiment of the Production Process

In the first embodiment of the process for producing the Al alloy sheet for press forming in the present invention, the process includes the following steps in this order: a casting step of casting an Al alloy containing 0.4 to 1.5 mass % of Si and 0.3 to 1.0 mass % of Mg, with the remainder being Al and inevitable impurities; a homogenizing heat treatment step of subjecting the slab of the Al alloy to a homogenizing heat treatment; a hot rolling step of subjecting to hot-rolling under the condition that a hot rolling ending temperature is 300° C. or lower; an annealing step of subjecting to annealing at a temperature of 300 to 500° C.; a cold rolling step of subjecting to cold-rolling at a cold rolling ending temperature of 100° C. or lower; a solution treatment step of treating at a temperature of 500° C. or higher; and a heating step of heating to a temperature of 70° C. or higher.

In the first embodiment of the process for producing the Al alloy sheet for press forming in the present invention, steps other than those described below may be additionally performed so long as such additional steps do not lessen the effects of the present invention. For example, steps such as washing, slitter processing for intermediate trimming or splitting, and leveler correction may be added as middle steps. With respect to steps other than those especially described below and to conditions therefor, it is possible to produce in ordinary ways. The conditions of each step are explained below while referring to FIG. 1.

Casting Step S1

The casting step S1 is a step in which an Al alloy for press forming is melted and cast to produce an Al alloy slab. In the casting step, a slab having a given shape is produced from a melt obtained by melting an Al alloy having the composition described above. Methods for melting and casting the Al alloy are not particularly limited, and a conventional method may be used. For example, a method in which the alloy is melted using an induction melting furnace, reverberatory melting furnace or the like and the melt is cast using a continuous casting method or a semi-continuous casting method can be used for casting.

Homogenizing Heat Treatment Step S2

The homogenizing heat treatment step S2 is conducted in order to render the slab even in structure as a whole since the slab obtained by mere casting is uneven in structure. The starting temperature of the homogenizing heat treatment is preferably 500 to 580° C. In a case where the temperature is below 500° C., much time is required for the structure to become even, resulting in a decrease in production efficiency. In a case where the temperature exceeds 580° C., local fusion may occur due to the lowered melting point of segregated portions.

It is preferable that the period of the homogenizing heat treatment should be 1 to 10 hours. In a case where the period of the homogenizing heat treatment is less than 1 hour, there is a possibility that the segregation might remain. Meanwhile, in a case where the period thereof exceeds 10 hours, the production efficiency decreases.

Hot Rolling Step S3

The hot rolling step S3 is a step in which after the homogenizing heat treatment step S2, the slab is hot-rolled in order to regulate the thickness thereof to a given value. The hot rolling is repeatedly performed in the course of temperature declining, until it comes to have a given thickness. The hot-rolling starting temperature is preferably 400 to 550° C. From the standpoint of reducing the plate thickness to a given value with a minimum number of rolling operations, the rolling is conducted at a high temperature. In a case where the hot-rolling starting temperature is low, the deformation resistance is so high that the rolling is difficult. Meanwhile, in a case where the hot-rolling starting temperature is too high, coarse grains formed by recrystallization are caused on the surface, and rough surface in the final product is caused thereby.

The hot rolling can be conducted so that the one-pass hot rolling reduction rate (rolling reduction) is in the range of about 30 to 50%, as in the general hot rolling of aluminum materials. It is preferable that the hot rolling reduction should be 30 to 40%. This is because by conducting the hot rolling so as to result in a reduction within that range, the quantity of heat generated during hot rolling is reduced and strain accumulation is increased.

It is necessary that the ending temperature in the finishing step of the hot rolling should be 300° C. or lower. The ending temperature is more preferably 170 to 290° C. In a case where the hot rolling ending temperature exceeds 300° C., strain accumulation is insufficient and, hence, the annealing step does not result in fine recrystallization but results in grain growth only in specific crystal orientations. As a result, the Al sheet is apt to be deformed only in limited directions and cannot have a structure with excellent isotropy.

Annealing Step S4

The annealing step S4 is a step in which annealing is conducted. Since the hot rolling ending temperature in the finishing step of the hot rolling step S3 is 300° C. or lower, a strain has accumulated in the grain structure inside the Al sheet. By eliminating the strain while keeping the Al sheet free from constriction in the annealing step S4, the grain structure inside the Al sheet can be made to have little strain in any direction and have high isotropy.

It is necessary that the annealing temperature should be 300 to 500° C. In a case where the annealing temperature is lower than 300° C., there is a possibility that no recrystallization might occur. In a case where the annealing temperature exceeds 500° C., there is a possibility that grain might be coarsened. It is preferable that the period of annealing should be longer than 0 second and 30 seconds or less in the case of a continuous furnace, and be 5 hours or less in the case of a batch furnace. Too long annealing periods result in the formation of coarse grain and increased anisotropy. It is preferable that a continuous furnace, in which a high temperature-rising rate is attained and fine recrystallization is hence apt to occur, should be used and the temperature-rising rate be regulated to 1° C./sec or higher.

Cold Rolling Step S5

The cold rolling step S5 is a step in which cold-rolling is conducted. After completion of the annealing step S4, cold rolling is conducted once or multiple times to a desired final sheet thickness. It is preferable that the cold rolling reduction rate should be 40% or higher. In a case where the cold rolling reduction rate is less than 40%, there are cases where the effect of forming fine grains during the solution treatment is not sufficiently obtained. The cold rolling ending temperature must be 100° C. or lower, and is preferably 80° C. or lower. In, a case where the cold rolling ending temperature is high, strain accumulation is insufficient and, hence, the solution treatment step does not result in fine recrystallization but results in grain growth only in specific crystal orientations. As a result, the Al sheet is apt to deform only in limited directions and cannot have an isotropic structure. The term “cold rolling ending temperature” means the temperature at which the final cold rolling ends in the case where cold rolling is performed multiple times.

After completion of the cold rolling described above, cold rolling at a low reduction rate may be performed, such as skin pass rolling for sheet flatness correction or rolling with an EDT (electric discharge textured) roll for surface roughness regulation.

Solution Treatment Step S6

The solution treatment step S6 is a step which is necessary for causing the Mg and Si to be dissolved and for ensuring proof stress after baking. With respect to solution treatment temperature, it is necessary to conduct the treatment at a temperature of 500° C. or higher, and the temperature is preferably 500 to 570° C. In a case where the solution treatment temperature is below 500° C., there is a possibility that the amount of a solid solution might be insufficient. In a case where the solution treatment temperature exceeds 570° C., there is the possibility that eutectic fusion may occur or recrystallized grains may be coarsened. It is preferable that the period of the solution treatment should be longer than 0 second and 60 seconds or less. In a case where the solution treatment period is too long, profitability is impaired since the effect is saturated. With respect to cooling after the temperature is reached to the heating temperature, low cooling rates are prone to result in precipitation of coarse grains of Mg₂Si, Si, etc. at grain boundaries, resulting in reduced formability. It is therefore preferred to cool by water cooling (water quenching) or the like.

Heating Step S7

The heating step S7 is a step for reducing the amount of change due to room-temperature aging and for ensuring proof stress after baking. The heating temperature must be 70° C. or higher, and is preferably 70 to 150° C. In a case where it is held at a temperature below 70° C., the increase in strength after baking is insufficient. In a case there the heating temperature exceeds 150° C., an initial strength is too high and hence formability is impaired.

Second Embodiment of the Production Process

In the second embodiment of the process for producing the Al alloy sheet for press forming in the present invention, the process includes the following steps in this order: a casting step of casting an Al alloy containing 0.4 to 1.5 mass % of Si and 0.3 to 1.0 mass % of Mg, with the remainder being Al and inevitable impurities; a homogenizing heat treatment step of subjecting the slab of the Al alloy to a homogenizing heat treatment; a hot rolling step of subjecting to hot-rolling; a first cold rolling step of subjecting to cold-rolling at a cold rolling ending temperature of 100° C. or lower; an intermediate annealing step of subjecting to intermediate annealing at a temperature of 300 to 500° C.; a second cold rolling step of subjecting to cold-rolling at a cold rolling ending temperature of 100° C. or lower; a solution treatment step of treating at a temperature of 500° C. or higher; and a heating step of heating to a temperature of 70° C. or higher.

In the second embodiment of the process for producing the Al alloy sheet for press forming in the present invention, steps other than those described below may be additionally performed so long as such additional steps do not lessen the effects of the present invention. For example, steps such as washing, slitter processing for intermediate trimming or splitting, and leveler correction may be added as middle steps. With respect to such steps other than those especially described below and to conditions therefor, it is possible to produce in ordinary ways. The conditions of each step are explained below while referring to FIG. 2.

In the second embodiment of the process for production, the casting step S1, homogenizing heat treatment step S2, solution treatment step S6, and heating step S7 are conducted under conditions which are common with those in the first embodiment of the process for production. Explanations of these steps are hence omitted.

Hot Rolling Step S3

In the second embodiment of the process for production, the hot rolling reduction rate (rolling reduction) and starting temperature in the hot rolling are the same as those in the first embodiment. There is no particular upper limit on the ending temperature in the finishing step of the hot rolling. From the standpoint of production efficiency, however, the ending temperature is preferably 400° C. or lower.

First Cold Rolling Step S5 a

This cold rolling step S5 a is a step in which after the hot rolling step S3, cold-rolling is conducted. After completion of the hot rolling step S3, cold rolling is conducted once or multiple times to a desired final sheet thickness. The cold rolling reduction rate is preferably 40% or higher, more preferably 50% or higher. The cold rolling ending temperature must be 100° C. or lower, and is preferably 80° C. or lower. In a case where the reduction rate or the ending temperature is outside the range, a finely recrystallized structure is not obtained in the intermediate annealing step.

Intermediate Annealing Step S4 a

The intermediate annealing step S4 a is a step in which after the first cold rolling step S5 a, intermediate annealing is conducted. In the first cold rolling step S5 a, a strain has been accumulated in the grain structure inside the Al sheet. By relieving the strain while keeping the Al sheet free from constriction in the intermediate annealing step S4 a, the grain structure inside the Al sheet can be made to have little strain in any direction and have high isotropy.

It is necessary that the intermediate annealing temperature should be 300 to 500° C. In a case where the intermediate annealing temperature is lower than 300° C., there is a possibility that no recrystallization might occur. In a case where the intermediate annealing temperature exceeds 500° C., there is a possibility that coarse grains might be formed. It is preferable that the period of intermediate annealing should be longer than 0 second and 30 seconds or less in the case of a continuous furnace, and be 5 hours or less in the case of a batch furnace. Too long annealing periods result in the formation of coarse grain and increased anisotropy. It is preferable that a continuous furnace, in which a high temperature-rising rate is attained and fine recrystallization is hence apt to occur, should be used and the temperature-rising rate be regulated to 1° C./sec or higher.

Second Cold Rolling Step S5 b

The second cold rolling step S5 b is a step in which after the intermediate annealing step S4 a, cold-rolling is conducted. After the completion of the annealing step S4, cold rolling is conducted once or multiple times to a desired final sheet thickness. It is preferable that the cold rolling reduction rate should be 40% or higher. In a case where the cold rolling reduction rate is less than 40%, there are cases where the effect of forming fine grains during the solution treatment is not sufficiently obtained. The cold rolling ending temperature must be 100° C. or lower, and is preferably 80° C. or lower. In a case where the cold rolling ending temperature is too high, strain accumulation is insufficient and, hence, the solution treatment step does not result in fine recrystallization but results in grain growth only in specific crystal orientations. As a result, the Al sheet is apt to deform only in limited directions and cannot have an isotropic structure. The term “cold rolling ending temperature” means the temperature at which the final cold rolling ends in the case where cold rolling is performed multiple times.

After completion of the cold rolling described above, cold rolling at a low reduction rate may be performed, such as skin pass rolling for sheet flatness correction or rolling with an EDT (electric discharge textured) roll for surface roughness regulation.

The Al alloy sheet for press forming obtained through production steps including the steps described above can be an Al alloy sheet for press forming which has excellent press formability.

EXAMPLES

The present invention is explained below on the basis of Examples. The present invention should not be construed as being limited to the following Examples.

ample Nos. 1 to 27

Sample Nos. 1 to 27 are each aluminum alloy sheet produced by the first embodiment of the process for production.

Al alloys (alloy symbols A to Z) having the compositions shown in Table 1, which will be given later, were melted and cast to obtain slabs having a thickness of 600 mm, by a conventional method such as a DC casting method. These slabs were subjected to a homogenizing heat treatment under the condition of 550° C. and 5 hours. Of the heat-treated slabs, those for sample Nos. 1 to 25 and sample No. 27 were repeatedly hot-rolled at a rolling reduction of 30 to 40% and at a hot-rolling starting temperature of 500° C. and were thereby gradually reduced in thickness. Thus, hot-rolled sheets having a sheet thickness of 3 mm were obtained through the hot rolling, in which the hot rolling ending temperature was 270° C. With respect to sample No. 26, a hot-rolled sheet having a sheet thickness of 3 mm was obtained through the hot rolling in which the hot rolling ending temperature was changed to 285° C.

Next, with respect to sample Nos. 1 to 25, the sheets were subjected to annealing using a continuous furnace under the condition of 500° C. and 20 seconds. With respect to sample No. 26, the sheet was subjected to annealing using a continuous furnace under the condition of 350° C. and 20 seconds. With respect to sample No. 27, the sheet was subjected to annealing using a batch furnace under the condition of 400° C. and 4 hours. Thereafter, with respect to sample Nos. 1 to 27, cold rolling was conducted at a cold rolling reduction rate (rolling reduction) of 66% to obtain cold-rolled sheets having a sheet thickness of 1 mm, for which the cold rolling ending temperature was 90° C. Next, using a continuous furnace, the sheets were heated at a temperature-rising rate of 300 ° C./min and, at the time when the temperature had reached 550° C., the sheets were held for 20 seconds, thereby performing a solution treatment. Immediately thereafter, the sheets were introduced into room-temperature water, followed by quenching at a cooling rate of 100° C./sec or higher, thereby performing quench-hardening. Finally, the sheets were subjected to a heat treatment in which the sheets were held at 100° C. for 2 hours, and then, the sheets were annealed at 0.6° C./h, thereby obtaining specimens. For measuring the temperatures of the Al alloy sheets, digital thermometer TC-950, manufactured by Line Seiki Co., Ltd. (the same applies hereinafter) was used.

Sample Nos. 28 to 32

Sample Nos. 28 to 32 are each aluminum alloy sheet produced by the second embodiment of the process for production.

Of the Al alloys having the compositions shown in Table 1, which will be given later, the alloys having the compositions indicated by alloy symbols A, E, and M were melted and cast to obtain slabs having a thickness of 600 mm, by a conventional casting method such as a DC casting method in the same manner as for sample Nos. 1, 5, and 13. These slabs were subjected to a homogenizing heat treatment under the condition of 550° C. and 5 hours. Of the heat-treated slabs, those for sample Nos. 28 to 30 and sample No. 32 were repeatedly hot-rolled at droning reduction of 30 to 40% and at a hot-rolling starting temperature of 500° C. and were thereby gradually reduced in thickness. Thus, hot-rolled plates having a plate thickness of 7 mm were obtained through the hot rolling, in which the hot rolling ending temperature was 250° C. With respect to sample No. 31, a hot-rolled plate having a plate thickness of 7 mm was obtained through the hot rolling in which the hot rolling ending temperature was changed to 330° C.

Next, first cold rolling was conducted at a cold rolling reduction rate (rolling reduction) of 57% and at the cold rolling ending temperatures of 90° C. or lower that are shown in Table 2, which will be given later, to obtain cold-rolled sheets having a sheet thickness of 3 mm. Thereafter, with respect to sample Nos. 28 to 31, the sheets were subjected to intermediate annealing using a continuous furnace under the condition of 500° C. and 20 seconds. With respect to sample No. 32, the sheet was subjected to intermediate annealing using a batch furnace under the condition of 400° C. and 5 hours. Thereafter, with respect to sample Nos. 28 to 32, second cold rolling was conducted at a cold rolling reduction rate (rolling reduction) of 67% and at the cold rolling ending temperatures of 90° C. or less that are shown in Table 2, respectively, thereby obtaining cold-rolled sheets having a sheet thickness of 1 mm. Next, using a continuous furnace, the sheets were heated at a temperature-rising rate of 300° C./min and, at the time when the temperature had reached 550° C., the sheets were held for 20 seconds, thereby performing a solution treatment. Immediately thereafter, the sheets were introduced into room-temperature water, followed by quenching at a cooling rate of 100° C./sec or higher, thereby performing quench-hardening. Finally, the sheets were subjected to a heat treatment in which the sheets were held at 100° C. for 2 hours, and then, they were annealed at 0.6° C./h, thereby obtaining specimens.

Sample Nos. 33 to 40

Sample No. 33 was obtained through processing under the same conditions as for sample No. 30, except that the cold rolling ending temperature in the first and second cold rolling was changed to 120° C.

Sample No. 34 was obtained through processing under the same conditions as for sample No. 30, except that the cold rolling ending temperature in the first cold rolling was changed to 120° C.

Sample No. 35 was obtained through processing under the same conditions as for sample No. 30, except that the cold rolling ending temperature in the second cold rolling was changed to 120° C.

Sample No. 36 was obtained through processing under the same conditions as for sample No. 13, except that the ending temperature in the hot rolling step was changed to 330° C.

Sample No. 37 was obtained through processing under the same conditions as for sample No. 13, except that the cold rolling ending temperature was changed to 110° C.

Sample No. 38 was obtained through processing under the same conditions as for sample No. 30, except that the intermediate annealing step was not conducted.

Sample No. 39 was obtained through processing under the same conditions as for sample No. 1, except that the ending temperature in the hot rolling step was changed to 250° C. and that a batch furnace was used to conduct annealing under the condition of 280° C. and 4 hours.

Sample No. 40 was obtained through processing under the same conditions as for sample No. 5, except that the ending temperature in the hot rolling step was changed to 250° C. and the annealing temperature was changed to 600° C.

With respect to each sample, the Al alloy sheet obtained through the heating step was allowed to stand for 3 months and then evaluated for properties under the following conditions.

Ratio Between the Diagonal Lengths of Indentations

A method for determining the ratio between the diagonal lengths of indentations is explained below using drawings. FIGS. 3 to 5 are schematic views for illustrating a method for measuring the length L0 of a diagonal that form an angle of 0° or 90° with the rolling direction and the length L45 of a diagonal that form an angle of 45° or −45° (135°) with the rolling direction, among the diagonals of indentations impressed with a Vickers hardness tester.

FIG. 3 shows an example of portions where indentations are impressed. A sample is taken out of the width-direction center of a sheet, and indentations having a substantially square shape are impressed with a Vickers hardness tester on the center of a cross-section of the sample sheet along the rolling direction (RD direction), so that at least three indentations (A1 to A3) are impressed in the case where the diagonals form an angle of 0° or 90° with the rolling direction and at least three indentations (B1 to B3) are impressed in the case where the diagonals form an angle of 45° or −45° (135°) with the rolling direction. In this impression, the load of the Vickers hardness tester was set at 100 g.

Using Vickers hardness measuring device AAV-500, manufactured by Mitsutoyo Corporation, the indentations are impressed on a cross-section of a specimen which has a thickness of 1 mm and which has been allowed to stand for 3 months after the heating step. This impressed cross-section is photographed using the automatic focusing function of the microscope united with the device.

FIG. 4 and FIG. 5 each show an example of methods for measuring the diagonal length on the photograph of indentations. On a photograph of one indentation, the two diagonal lengths are measured. FIG. 4 shows the case where the diagonals of an indentation form an angle of 0° or 90° with the rolling direction. An average of measured lengths a1 and a2 is used as the length L0 of the diagonals which form angles of 0° and 90° with the rolling direction (RD direction). FIG. 5 shows the case where the diagonals of an indentation form an angle of +45° or −45° (135°) with the direction of the rolling direction. An average of measured lengths b1 and b2 is used as the length L45 of the diagonals which form angles of 45° and −45° (135°) with the rolling direction (RD direction). In each case, the measurement is made on at least three indentations, and an average value of the diagonal lengths obtained is calculated.

The difference ΔL between the length L0 of the diagonal which forms an angle of 0° with the rolling direction and the length L45 of the diagonal which forms an angle of 45° with the rolling direction is determined. The proportion P (%) of the difference ΔL between the two lengths to the length L0 of the diagonal which forms an angle of 0° with the rolling direction is determined. In a case where this value was 2.0% or less, the sheet was judged to be low in anisotropy and excellent in formability.

Tensile Test

Tensile test pieces of the JIS No. 5 were punched out of a sheet specimen so that the rolling direction was the longitudinal direction. A tensile test was conducted with floor type universal tensile tester AG-1, manufactured by SHIMADZU CORPORATION, in accordance with JIS Z2241 to measure the tensile strength (MPa), tensile elongation (%), and 0.2% proof stress (MPa). The crosshead speed was 5 mm/min, and each test piece was stretched at the constant speed until the test piece ruptured. The measurement was made five times for each sheet specimen, and an average thereof was calculated. Tensile strengths of 210 MPa or higher were judged to be excellent, proof stresses of 120 MPa or higher were judged to be excellent, and tensile elongations of 20% or higher were judged to be excellent.

AB Proof Stress

AB proof stress is an index of BH properties (bake hardenability; paint baking hardenability) whereby strength and proof stress are improved by an artificial aging treatment, e.g., paint baking, after press forming. In a case where a formed article obtained through press forming of an Al alloy sheet is subjected to a relatively low-temperature treatment such as paint baking, the formed article undergoes age hardening due to the heating to improve the strength and proof stress. The degree of this improvement is expressed by the index.

As an artificial age hardening treatment, a heat treatment under the condition of 170° C. and 20 minutes was conducted in the state of previously giving a 2% strain (stretch) as a simulation of press forming. Thereafter, a tensile test was conducted with floor type universal tensile tester AG-1, manufactured by SHIMADZU CORPORATION, in accordance with JIS Z2241 to measure the 0.2% proof stress (AB proof stress) (MPa). The crosshead speed was 5 mm/min, and each specimen was stretched at the constant speed until the specimen ruptured. The measurement was made five times, and an average thereof was calculated. AB proof stresses of 170 MPa or higher were judged to be excellent.

Earing

A disk-shaped test piece (blank) having an outer diameter of 66 mm was punched out of a sheet specimen, and this test piece was subjected to cupping using a punch having a diameter of 40 mm to produce a drawn cup having a diameter of 40 mm. The height of each ear of this drawn cup was measured, and the earing (0°, 90° earing) (%) was determined on the basis of the following expression (2). In the expression (2), hX represents the height of an ear of the drawn cup. The numeral X affixed to the h indicates the position where a cup height measurement was made, and means the position which forms an angle of X° with the rolling direction of the Al alloy sheet.

Earing (%)=[{(h0+h90+h180+h270)−(h45+h135+h225 30 h315)}/{½ (h0+h90+h180+h270+h45+h135+h225+h315)}]×100  (2)

In a case where the earing is 3.5% or less, there is no large difference in deformation amount between the direction which forms an angle of 0° or 90° with the rolling direction and the direction which forms an angle of 45° therewith. Such cases were judged to be excellent in terms of formability and yield.

Stretch Formability

FIG. 6 is a cross-sectional view for illustrating a measurement method by a stretch formability tester.

In place of the evaluation of an aluminum alloy sheet for cracks by press working, critical forming height in sphere-head stretch forming was evaluated. A sheet specimen 13 was cut out so as to have dimensions of 110 mm (rolling-direction length)×200 mm (length along direction perpendicular to rolling direction). As shown in FIG. 6, this sheet specimen 13 was fixed to a die 10 having an inner diameter (hole diameter) of 102.8 mm, a shoulder radius Rd of 5.0 mm, and an outer diameter of 220 mm, using a jig (blank holder) 11 at a given hold-down force. While keeping the space between the die and the jig constant by sandwiching therebetween a shim (not shown) having the same thickness of 1 mm as the test piece, a sphere-head punch 12 having a sphere-head diameter of 100 mm (radius Rp: 50 mm) was pushed perpendicularly into the surface of the sheet specimen to conduct stretch working. The critical value of forming height H which was reached before a crack or constriction was observed was determined. The sheet specimens which showed a critical value of forming height H of 30 mm or larger were judged to be acceptable.

Inhibition of Ridging Marks

A test piece was cut out of a specimen so that the dimension thereof in the direction forming an angle of 0° with the rolling direction of the specimen was 40 mm and the dimension thereof in the direction forming an angle of 90° therewith was 200 mm. A 15% plastic strain was given thereto in the direction forming an angle of 90° with the rolling direction. Thereafter, the test piece was subjected, as a simulation of the painting of automotive body panels, to a treatment with zinc phosphate and then cationic electrodeposition, and was further subjected, as a simulation of paint baking hardening, to an annealing treatment. The surface of this sheet was then visually observed and evaluated. The specific treatment conditions are as follows. The sheet to which the strain had been given beforehand was subjected successively to a treatment with a colloidal dispersion of titanium phosphate and a zinc phosphate treatment in which the sheet was immersed in a zinc phosphate bath containing fluorine in a low concentration (50 ppm), thereby forming a zinc phosphate coating film on the sheet surface. This sheet was further subjected to cationic electrodeposition and then subjected to a heat treatment at 170° C. for 20 minutes.

The sheets in which the paint surface had no ridging marks were judged to be “oo”, those in which the paint surface had ridging marks relatively slightly were judged to be “o”, and those in which the paint surface had considerable ridging marks were judged to be “−”.

The results of evaluation of sample Nos. 1 to 25 are shown in Table 1. The results of evaluation of sample Nos. 1, 5, 13, and 26 to 40 are shown in Table 2. In the column “Alloy composition” in Table 1, “−” indicates that the content of the composition was below the detection limit for the analyzer. Furthermore, of the numerals shown in Table 1 and Table 2, the underlined numerals are numerals which are outside the numerical range shown in claim 1 or claim 2 or 3. Moreover, the sample Nos. 1, 5, and 13 in Table 2 are identical with the sample Nos. 1, 5, and 13 in Table 1.

TABLE 1 Alloy composition (remainder: Al Sample Alloy and inevitable impurities) (mass %) Proportion No. symbol Si Mg Cu Fe Mn Cr Zr Ti Zn P (%) 1 A 1.4 1.0 — 0.40 0.40 — — — — 1.3 2 B 0.8 0.7 — — — — — — — 1.8 3 C 1.0 0.6 0.15 — — — — — — 1.7 4 D 1.0 0.6 — 0.18 0.14 — — — — 1.7 5 E 0.6 0.6 0.15 0.18 0.14 — — — 0.1 1.5 6 F 1.0 0.6 — 0.18 — 0.1 — 0.05 0.4 1.4 7 G 1.0 0.6 — 0.18 — 0.1 — — — 1.6 8 H 1.0 0.6 — 0.22 — — — — 0.4 1.6 9 I 1.0 0.6 — 0.18 — — — 0.05 — 1.6 10 J 1.0 0.6 — 0.18 — 0.1 — — 0.4 1.5 11 K 1.0 0.6 — 0.18 — — — 0.05 0.4 1.5 12 L 1.0 0.6 — 0.18 — 0.1 — 0.05 — 1.5 13 M 1.0 0.4 0.15 0.18 0.14 — — — — 1.6 14 N 1.0 0.5 0.70 0.18 — — — — — 1.4 15 O 1.0 0.6 — 0.18 — — 0.1 — — 1.7 16 P 2.0 0.6 — 0.18 — — — — — 1.3 17 Q 0.2 0.6 — 0.18 — — — — — 2.5 18 S 1.0 0.6 — 1.0  — — — — — 1.2 19 T 1.0 0.6 — 0.18 1.0  — — — — 1.1 20 U 1.0 1.5 — 0.18 — — — — — 0.9 21 V 1.0 0.2 — 0.18 — — — — — 2.6 22 W 1.0 0.6 — 0.18 — 0.8 — — — 1.2 23 X 1.0 0.6 — 0.18 — — — — 1.0 1.3 24 Y 1.0 0.6 — 0.18 — — — 0.7  — 1.2 25 Z 1.0 0.6 — 0.18 — — 0.8 — — 1.2 Tensile Tensile AB proof Sample Alloy strength Proof stress elongation stress Earing Protrusion height No. symbol (MPa) (MPa) (%) (MPa) (%) (mm) 1 A 280 165 21 215 2.9 32 2 B 250 139 25 182 3.2 34 3 C 245 130 29 196 3.4 36 4 D 238 130 28 196 3.3 35 5 E 228 132 26 180 3.1 37 6 F 253 138 25 204 3.1 37 7 G 239 131 27 197 3.3 35 8 H 240 132 27 198 3.3 35 9 I 247 132 27 198 3.3 35 10 J 249 134 26 199 3.2 36 11 K 244 136 26 201 3.2 36 12 L 245 137 26 203 3.2 36 13 M 252 136 29 195 3.2 35 14 N 265 152 28 236 3.0 37 15 O 237 129 26 194 3.4 34 16 P 300 185 24 245 2.8 29 17 Q 210 95 29 155 3.7 27 18 S 190 73 15 155 2.6 29 19 T 250 139 24 205 2.4 27 20 U 340 225 19 285 2.2 26 21 V 165 95 33 105 3.8 26 22 W 244 139 26 205 2.5 28 23 X 249 141 25 207 2.7 25 24 Y 243 138 27 202 2.5 27 25 Z 242 137 25 203 2.4 27

TABLE 2 First Second Hot rolling cold rolling Kind of Cold rolling cold rolling ending ending furnace in Annealing ending ending Sample Alloy Production temperature temperature annealing temperature temperature temperature No. symbol process (° C.) (° C.) Annealing step step (° C.) (° C.) (° C.) 1 A first 270 — annealing continuous 500 90 — 5 E first 270 — annealing continuous 500 90 — 13 M first 270 — annealing continuous 500 90 — 26 M first 285 — annealing continuous 350 90 — 27 M first 270 — annealing batch 400 90 — 28 A second 250 80 intermediate annealing continuous 500 — 80 29 E second 250 80 intermediate annealing continuous 500 — 80 30 M second 250 80 intermediate annealing continuous 500 — 80 31 M second 330 70 intermediate annealing continuous 500 — 70 32 M second 250 80 intermediate annealing batch 400 — 80 33 M second 250 120  intermediate annealing continuous 500 — 120  34 M second 250 120  intermediate annealing continuous 500 — 80 35 M second 250 80 intermediate annealing continuous 500 — 120  36 M first 330 — annealing continuous 500 90 — 37 M first 270 — annealing continuous 500 110  — 38 M second 250 80 none — — — 80 39 A first 250 — annealing batch 280 90 — 40 E first 250 — annealing continuous 600 — — Tensile Tensile AB Forming Sample Alloy Proportion strength Proof stress elongation Proof stress Earing height Inhibition of No. symbol P (%) (MPa) (MPa) (%) (MPa) (%) (mm) ridging marks 1 A 1.3 280 165 21 215 2.9 32 ∘∘ 5 E 1.5 228 132 26 180 3.1 37 ∘∘ 13 M 1.6 252 136 29 195 3.2 35 ∘∘ 26 M 1.7 251 135 29 194 3.2 35 ∘ 27 M 1.8 250 134 28 193 3.3 34 ∘ 28 A 1.1 285 170 20 220 2.7 34 ∘∘ 29 E 1.3 233 137 25 185 3.0 39 ∘∘ 30 M 1.4 257 141 28 200 3.1 37 ∘∘ 31 M 1.5 254 139 22 190 3.1 38 ∘∘ 32 M 1.7 255 139 27 198 3.2 36 ∘ 33 M 2.4 248 129 25 190 3.6 30 x 34 M 2.2 253 137 24 196 3.3 33 x 35 M 2.3 249 130 24 191 3.6 31 x 36 M 2.6 241 124 21 183 3.8 28 x 37 M 2.2 250 134 28 192 3.3 33 x 38 M 3.1 239 125 29 179 4.5 25 x 39 A 3.0 237 123 29 177 4.5 26 x 40 E unable to be evaluated because of melting during annealing

As shown in Table 1, the Al alloy sheets for press forming (sample Nos. 1 to 15) constituted of Al alloys which satisfied the requirements concerning alloy composition in the present invention had excellent performance in terms of all of tensile strength, proof stress, tensile elongation, AB proof stress, earing, and forming height. Meanwhile, the Al alloy sheets for press forming (sample Nos. 16 to 25) constituted of Al alloys which did not satisfy the requirements in the present invention were each poor in forming height. Furthermore, sample Nos. 17, 18, 20, and 21 were poor in any one or more of tensile strength, proof stress, tensile elongation, AB proof stress, and earing.

As shown in Table 2, the Al alloy sheets for press forming (sample Nos. 1, 5, 13, and 26 to 32) which were constituted of Al alloys and satisfied the requirements concerning production process in the present invention had excellent performance in terms of all of tensile strength, proof stress, tensile elongation, AB proof stress, earing, forming height, and inhibition of ridging marks. In particular, by employing production conditions in which the cold rolling ending temperature had been regulated to 100° C. or lower and a step for giving an annealing step or intermediate annealing step had been included, the performance concerning forming height, etc. was further improved. Meanwhile, sample Nos. 13 and 27 and sample Nos. 30 and 32 are each show a comparison between use of a continuous furnace and use of a batch furnace in an annealing step. In either case, use of the continuous furnace was able to give an Al alloy sheet for press forming which had better performance.

On the other hand, the Al alloy sheets for press forming (sample Nos. 33 to 39) which were constituted of Al alloys and satisfied the Al alloy composition but did not satisfy the requirements concerning production conditions in the present invention each had a proportion P exceeding 2.0% and were poor in any one or more of earing, forming height, and inhibition of ridging marks. Sample No. 40 melted during the annealing because of the too high annealing temperature, and no sample for evaluation was able to be obtained therefrom. Sample Nos. 33 to 35 had undergone insufficient strain accumulation since either one or both of the first cold rolling ending temperature and the second cold rolling ending temperature in the second embodiment of the process for production had exceeded 100° C., and each showed a proportion P exceeding 2.0% and insufficient isotropy. Sample No. 36 had undergone insufficient strain accumulation and not undergone fine recrystallization since the ending temperature in the hot rolling step in the first embodiment of the process for production had exceeded 300° C., and hence showed a proportion P exceeding 2.0% and insufficient isotropy. Sample No. 37 had undergone insufficient strain accumulation and not undergone fine recrystallization since the cold rolling ending temperature in the first embodiment of the process for production had exceeded 100° C., and hence showed a proportion P higher than 2.0% and insufficient isotropy. Sample No. 38 was a sample produced without performing any annealing step, had not undergone fine recrystallization, and showed a proportion P exceeding 2.0% and insufficient isotropy. Sample No. 39 had not undergone fine recrystallization since the annealing temperature in the first embodiment of the process for production had been below 300° C., and showed a proportion P exceeding 2.0% and insufficient isotropy.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

This application is based on a Japanese patent application No. 2013-081149 filed on Apr. 9, 2013 and a Japanese patent application No. 2014-055180 filed on Mar. 18, 2014, the contents of which are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The aluminum alloy sheet in the present invention is useful as a material for automotive exterior sheet materials for bodies, doors, fenders, etc., and has excellent formability which renders the alloy sheet applicable even to deep press forming.

DESCRIPTION OF REFERENCE NUMBERS

-   S1 Casting step -   S2 Homogenizing heat treatment step -   S3 Hot rolling step -   S4 Annealing step -   S4 a Intermediate annealing step -   S5 Cold rolling step -   S5 a First cold rolling step -   S5 b Second cold rolling step -   S6 Solution treatment step -   S7 Heating step 

1. An aluminum alloy sheet for press forming, comprising: an aluminum alloy comprising 0.4 to 1.5 mass % of Si and 0.3 to 1.0 mass % of Mg, with the remainder being Al and inevitable impurities, wherein with respect to diagonal lengths of indentations formed therein with a Vickers hardness tester, a proportion P (%) of a difference ΔL between a length L0 of a diagonal which forms an angle of 0° with a rolling direction and a length L45 of a diagonal which forms an angle of 45° with the rolling direction to the L0 is 2.0% or less.
 2. The aluminum alloy sheet for press forming according to claim 1, wherein the aluminum alloy further comprises at least one of the following: 1.0 mass % or less of Cu; at least one of 0.5 mass % or less of Fe and 0.5 mass % or less of Mn; at least one of 0.3 mass % or less of Cr, 0.3 mass % or less of Zr, and 0.3 mass % or less of Ti; and 0.5 mass % or less of Zn.
 3. The aluminum alloy sheet for press forming according to claim 1, wherein the aluminum alloy comprises 0.6 mass % to 1.3 mass % of Si and 0.3 mass % to 0.8 mass % of Mg.
 4. An aluminum alloy press-formed article produced by press-forming the aluminum alloy sheet according to claim
 1. 5. An aluminum alloy press-formed article produced by press-forming the aluminum alloy sheet according to claim
 3. 6. A process for producing an aluminum alloy sheet, comprising: casting an aluminum alloy comprising 0.4 mass % to 1.5 mass % of Si and 0.3 mass % to 1.0 mass % of Mg, with the remainder being Al and inevitable impurities; subjecting a slab of the aluminum alloy to a homogenizing heat treatment; hot-rolling the aluminum alloy, at a hot rolling ending temperature of 300° C. or lower; annealing the aluminum alloy at a temperature of 300° C. to 500° C., cold-rolling the aluminum alloy at a cold rolling ending temperature of 100° C. or lower; subjecting the aluminum alloy to a solution treatment step at a temperature of 500° C. or higher; and heating the aluminum alloy to a temperature of 70° C. or higher.
 7. A process for producing an aluminum alloy sheet for press forming, comprising: casting an aluminum alloy comprising 0.4 mass % to 1.5 mass % of Si and 0.3 mass % to 1.0 mass % of Mg, with the remainder being Al and inevitable impurities; subjecting a slab of the aluminum alloy to a homogenizing heat treatment; hot-rolling the aluminum alloy; cold-rolling the aluminum alloy at a cold rolling ending temperature of 100° C. or lower; intermediate annealing the aluminum alloy at a temperature of 300° C. to 500° C.; cold-rolling the aluminum alloy at a cold rolling ending temperature of 100° C. or lower; subjecting the aluminum alloy to a solution treatment at a temperature of 500° C. or higher; and heating the aluminum alloy to a temperature of 70° C. or higher.
 8. The process for producing an aluminum alloy sheet for press forming according to claim 6, wherein the aluminum alloy further comprises at least one of the following: 1.0 mass % or less of Cu; at least one of 0.5 mass % or less of Fe and 0.5 mass % or less of Mn; at least one of 0.3 mass % or less of Cr, 0.3 mass % or less of Zr, and 0.3 mass % or less of Ti; and 0.5 mass % or less of Zn.
 9. The process for producing an aluminum alloy sheet for press forming according to claim 6, wherein the aluminum alloy comprises 0.6 mass % to 1.3 mass % of Si and 0.3 mass % to 0.8 mass % of Mg, with the remainder being Al and inevitable impurities.
 10. The process for producing an aluminum alloy sheet for press forming according to claim 8, wherein the aluminum alloy comprises 0.6 mass % to 1.3 mass % of Si and 0.3 mass % to 0.8 mass % of Mg, with the remainder being Al and inevitable impurities.
 11. The aluminum alloy sheet for press forming according to claim 2, wherein the aluminum alloy comprises 0.6 mass % to 1.3 mass % of Si and 0.3 mass % to 0.8 mass % of Mg, with the remainder being Al and inevitable impurities.
 12. An aluminum alloy press-formed article produced by press-forming the aluminum alloy sheet for press forming according to claim
 2. 13. An aluminum alloy press-formed article produced by press-forming the aluminum alloy sheet for press forming according to claim
 11. 14. The process for producing an aluminum alloy sheet for press forming according to claim 7, wherein the aluminum alloy further comprises at least one of the following: 1.0 mass % or less of Cu; at least one of 0.5 mass % or less of Fe and 0.5 mass % or less of Mn; at least one of 0.3 mass % or less of Cr, 0.3 mass % or less of Zr, and 0.3 mass % or less of Ti; and 0.5 mass % or less of Zn. 